In general, rotorcraft rotors comprise a hub that is driven in rotation about an axis of rotation by an outlet shaft from a power transmission gearbox, referred to as the drive shaft, together with at least two blades that are fastened to the hub via suitable hinges, in particular via a respective laminated spherical thrust-bearing dedicated to each blade, together with inter-blade dampers, each interconnecting two adjacent blades, or dampers connecting each blade to the hub.
Assuming that each blade is engaged in a hub so as to be restrained in bending, the rotor constituted in this way is a rigid rotor. When hovering, the distribution of aerodynamic forces along a blade gives rise to a distribution of bending moments in flapping and in drag, which bending moments are of values that are very large at the root of the blade because of the increase in the circumferential speed proportional to the radius of the rotor.
Furthermore, when flying in translation, the so-called “advancing” blade generates more lift than the so-called “retreating” blade because of the difference in their air speeds, as described in greater detail below.
Consequently, the resultant of the aerodynamic forces exerted on a blade does not have the same value at each azimuth position, nor do the resultants have the same points of application: the restrained bending moment of the root of the blade is thus high and varying, thereby giving rise to alternating stresses that give rise to a fatigue phenomenon that is harmful to materials. In addition, the resultant of the aerodynamic forces of all of the blades is no longer directed along the axis of the rotor, thereby creating a roll moment, that increases with speed and that can make it difficult to balance forces when flying in translation.
In order to remedy those drawbacks, it is known to hinge the blades to the rotor about respective axes perpendicular to the drive shaft and referred to as axes for vertical flapping, corresponding to hinges for vertical flapping capable of transferring a force of arbitrary orientation but not capable under any circumstances of transferring a moment. Consequently, if a blade is hinged to flap relative to the hub, its bending moment in flapping at its point of attachment is zero. To enable the blade to be balanced, the centrifugal forces hold the blade in position after it has moved up a certain amount, thereby producing a cone of angle a0.
Under such conditions, there is firstly no longer any major roll moment when flying in translation, and secondly the rotating blades no longer describe a plane, but rather their outer ends describe a very flat cone.
In practice, the flapping axis no longer lies on the axis of rotation, but is offset therefrom by a distance a, referred to as its eccentricity.
It should also be recalled that in order to provide a helicopter with lift in its various configurations, it is necessary to be able to control the lift of the rotor and to vary it. That is why a pitch hinge is provided, of axis that is substantially parallel to the span of the corresponding blade. This new degree of freedom enables the lift of the blade to be controlled by acting on the general pitch control, and also enables pitch to be varied cyclically, thereby enabling the plane of rotation of the blades to be controlled so that they describe a cone of geometrical axis that no longer coincides with the drive axis: the resultant of the forces applied to the hub changes direction together with the plane of the rotor. Because of this, moments are generated about the center of gravity of the helicopter, thereby enabling it to be piloted.
As mentioned above, the plane of rotation of the blades may be other than a plane perpendicular to the drive shaft. Under such conditions, it is necessary for each blade to be hinged to pivot in drag since the end of each blade is at a variable distance from the rotor shaft. Otherwise, inertia forces would necessarily appear, thereby generating reciprocating bending movements on each blade in its own plane. Such a drag hinge allows the blade to pivot about a drag axis that is substantially parallel to the rotor axis, and consequently substantially perpendicular to the drag forces. To enable such a blade to be driven by the drive shaft, it is naturally necessary for the drag hinge to be far enough away from the rotor axis for the moment due to centrifugal forces to balance the moment due to drag and inertia forces, thereby requiring the drag axis to be offset or eccentric by an amount e, and this must be achieved without the so-called “drag” angle δ being too great.
Consequently, the blades of a hinged rotor for a rotary wing aircraft, in particular a helicopter, can be subjected to the following four kinds of movement:
i) rotation about the rotor axis;
ii) pivoting about the axis for vertical flapping, made possible by the vertical flapping hinge;
iii) pivoting about the drag axis, also known as the axis for horizontal flapping, made possible by the horizontal flapping hinge or drag hinge; and
iv) pivoting about the pitch axis of the blade, made possible by a pitch hinge (not specific to hinged rotors).
By way of example, provision is made in patent FR 2 497 073 for the three above-described pivoting movements II, III, and IV to be made possible by a single member such as a laminated spherical thrust-bearing.
Nevertheless, the oscillations of each blade about its drag axis can become coupled in unstable manner with the movements of the airframe or with its elastic deformation modes, in particular with oscillations of a helicopter that is standing on the ground on its landing gear: this is the origin of the so-called “ground resonance” phenomenon that can be dangerous for the rotorcraft when the resonant frequency of the oscillations of the blades about their drag axes and expressed relative to the frame of reference of the rotorcraft is close to one of the resonant frequencies of oscillation of the rotorcraft.
Document FR 791 701 discloses an inertial resonator carried by a rotor blade for damping or contributing to damping the vibration or the oscillation of said blade.
That inertial resonator comprises one or more heavy elements capable of performing transverse movements relative to the longitudinal axis of the blade.
Thereafter, a “box” is fastened to the rib of the blade. Since the rib of the blade extends along the longitudinal axis thereof, the box is arranged transversely to said rib.
At least one heavy element is then placed in the box, with the box acting as guide means therefor when it moves transversely.
According to that document FR 791 701, it is appropriate to move a heavy element transversely in order to solve the problem posed.
Document FR 791 701 explains that if the blade performs a drag movement, the weight moves in the direction opposite to the direction in which the blade moves, thereby contributing to damping the movement of the blade, the movement of the heavy element being retarded relative to the movement of the blade, because of its inertia.
Similarly, although the field of wind turbine blades is remote from the invention insofar as the phenomenon of ground resonance does not appear as such, documents DE 10 202 995 and EP 0 792 414 envisage inertial resonators making use of the transverse shifting of a heavy element in a direction perpendicular to a longitudinal direction of the blade, said longitudinal direction passing via the root of the blade and its end, and being substantially parallel to the axis for blade pitch variation, or indeed coinciding with said pitch variation axis.
Finally, on the same lines, document EP 1 101 034 provides for a wind turbine blade provided with an O-shaped cavity within which a liquid moves in the direction of oscillations, and thus transversely relative to the blade.
Although they are effective, those various resonators providing for a heavy element to move transversely provide damping that is limited, and therefore they do not give complete satisfaction.
Consequently, rotorcraft manufacturers generally make use of a different solution. Such manufacturers remedy the above-mentioned phenomenon of “ground resonance” by introducing on the drag axes damping by means of a resonator having a dry or viscous type damper, or indeed by introducing stiffness with the help of blade-spacing cables optionally associated with dampers, as for the Alouette helicopter made by the Applicant.
A function analogous to that of blade-spacing cables is provided by resilient inter-blade connections. In practice, this amounts to placing a damper between pairs of adjacent blades, the fastenings for such a damper to each of the two adjacent blades being at equal distance from the center of the rotor, i.e. on an identical radius from said rotor center.
Such inter-blade drag dampers include resilient return means of determined stiffness and damping for combating resonance phenomena, in particular ground resonance and also drive system resonance that can also appear, particularly on helicopters.
Patents FR 2 630 703 and U.S. Pat. No. 4,915,585 describe a rotor in which each blade is fastened to the hub by a sleeve having ends in the form of forks each comprising two spaced-apart and facing tines, with an inter-blade drag damper being fastened to two adjacent blades via two respective ball-joints.
Although effective, an arrangement of inter-blade drag dampers presents drawbacks.
Firstly, the weight of each inter-blade damper commonly lies in the range six to eleven kilograms, and that is not negligible.
Secondly, the movement of the blade in flight is forced to the frequency of the main rotor, so loads are imposed on the hub and on the portion of the blade or the sleeve serving to fasten the damper. These loads thus give rise to those various components being overdimensioned, and thus to an increase in the weight of the hub.
Finally, it should be observed that inter-blade dampers work most of the time in part under the effect of dynamic movements of the blades and they increase the aerodynamic drag of the rotor.